The present invention relates in general to a control system for controlling a blade carried by a motorgrader used for earthworking, and, more particularly, to a method and apparatus for controlling the spatial orientation of the blade of an earthmoving machine while shaping a surface of earth and, even more particularly, to a method and apparatus for controlling the cross-slope angle cut by a motorgrader while the motorgrader is making a turn.
Earthmoving machines for shaping the surface of the ground at a construction site typically include a frame mounting some form of an earthmoving or cutting blade. When preparing the subsurface of, for example, a highway, an airport runway, a parking lot and the like, it is typically desirable for the contour or grade of the subsurface shaped by the blade to approximate the finished surface as closely as possible. How accurately the surface of the ground is shaped depends upon how accurately the spatial orientation of the earthmoving blade can be determined and maintained and how accurately the direction of travel of the blade can be determined. The blade of some earthmoving machines are more difficult to accurately control than others.
For example, a typical motorgrader has a two-part articulated frame, defined by a rear drive unit and a front steering unit, and a cutting blade mounted on the front steering unit. The articulated frame allows the front steering unit to be rotated or pivoted relative to the drive unit. For example, the motorgrader is said to be in a "crabbed" steering position when it is operated in an articulated position and traveling in the direction defined by and in-line with its rear drive unit. It is often desirable to operate a motorgrader with its front steering unit articulated at an angle relative to its rear drive unit, for example, to position the drive unit on firm ground. As another example, the motorgrader is said to be steering through a turn when the front wheels on the steering unit are turned either to the right or left and the rear drive unit is either straight or turned to the same side as the front wheels. It is also desirable to cut a grade with a motorgrader while steering through a turn as would be the case in building a clover leaf on a ramp or a cul-de-sac. A motorgrader cutting blade is usually mounted on its steering unit so as to be adjustably moveable, including one or more being rotated about a central vertical axis, pitched forward or backward, rolled (i.e., banked) or side-shifted to the left or right and vertically raised or lowered.
The slope of a motorgrader blade is usually one element of the blade's spatial configuration that is controlled during the surface shaping process. By monitoring the direction of travel of the blade and monitoring its slope, the surface of the earth can be formed to a predetermined cross-slope. The definition of slope is the slant of a surface relative to horizontal. Cross-slope is defined as the slope of a surface perpendicular to the direction of travel. When a motorgrader is operated in a turning mode, the actual direction of travel of the blade is different than any other structural member of the blade. This fact combined with the ability of the frame to articulate and/or the blade circle to side-shift, can compound the already difficult task of accurately controlling the cross-slope of the cutting blade.
A number of systems have been used to control the spatial orientation (e.g., the azimuth, pitch, roll and/or elevation) of an earthmoving blade, including the cutting blade of a motorgrader. However, many of these control systems are relatively inaccurate, particularly when the machine frame mounting the blade is articulated, as often occurs in operating a motorgrader. There are more accurate control systems than these, but they are relatively complex and expensive. And, even these more accurate control systems are unable to maintain a high degree of accuracy when the machine is turning or the circle is side-shifted, because they have no way of sensing that these events are occurring. If there is no compensation for the rotational effects of turning or side-shifting then errors are introduced into the control system.
Accordingly, there is a need for a relatively simple and inexpensive system for more accurately controlling the spatial orientation (e.g., the azimuth, pitch, roll and/or elevation) of an earthmoving blade and, thereby, more accurately control the shaping of a surface of the ground at a work site. More particularly, there is a need for a relatively simple and inexpensive way to determine the direction of travel and orientation of an earthmoving blade relative to gravity and independent of the balance of the earthmoving machine to thereby control the shaping of a requested slope or cross-slope cut in the ground, even while the motorgrader is turning, the blade is rotated or side-shifted, the frame is articulated, or the front wheels are tilted.